Accurate measurement of the surface residual stresses generated by milling in pre-equilibrium state
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We introduce a method to measure accurately surface residual stresses in the pre-equilibrium state, which were generated in workpieces during the milling process. The method takes into account strain changes and uses the inverse calculation. Material in the stress layer was removed layer by layer, and the strain change on the opposite side of the machined surface was measured. We also consider the change of the bending moment caused by the changed neutral layer. The stress values were calculated from the last layer to the first layer, and the residual stresses generated by milling are measured. We created a finite element model of a real workpiece and the measured stress values were used as inputs for the model. The measuring method was validated using finite element analysis. We find that our measuring method can be successfully used in practice to measure surface residual stresses and it provides reliable indicators for evaluating the surface properties of machined workpieces.
I. INTRODUCTION
Residual stresses significantly affect fatigue life, geometrical stability, corrosion resistance and crack resistance of machined workpieces.1,2 Surface residual stresses occur due to severe inelastic (plastic) deformations caused by high temperatures, high pressures, high strain rates, thermal gradients, and phase transformations during the machining process.3 The depth of the stress layer is very shallow. Most of the time it is less than 0.2 mm, but the stress gradients within the stress layers are often very large. Since the 1930s, over ten different methods have been developed to measure residual stresses. The methods can be divided into two main categories: destructive and nondestructive. The destructive methods include centerhole drilling, ring-core, deep-hole, sectioning, and contour. Among these, the center-hole drilling method is most commonly used. The nondestructive techniques include the Barkhausen noise method, x-ray diffraction (XRD), neutron diffraction, and ultrasonic tests. The most commonly used among these is XRD.4–7 Based on the characteristics of surface residual stresses induced by machining, to date, the most commonly used measurement procedure involves a combination of the XRD method and layer removal.3,8 As Contributing Editor: Jürgen Eckert Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] c) e-mail: [email protected] DOI: 10.1557/jmr.2016.205
a result, they cannot include the initial states of the residual stresses induced by the machining process or predict the deformations of workpiece with different rigidities. This can be explained, as the final state of the surface residual stresses will go through two phases. First, the surface residual stresses in pre-equilibrium state are caused by many factors during the machining process. Second, the surface residual stresses are redistributed to achieve an equilibrium state, which will cause some deformations of the workpiece—see Fig. 1. The stress values obtained with the XRD method are the val
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